Just like with Windows (which if you'll notice when you CTRL/ALT/DEL to stop a program, it warns you that you may be making the system unstable) closing down programs manually can really screw things up. And with a Task killer, you are often doing that to a handful or programs all at once. Each OS has a system of cycles for it's programs, like right in the foreground, or further back if it falls idle, etc, and though the OS's don't always manage them perfectly, they are TRYING to manage them, and will do their best if left to it. If you go closing down programs when YOU feel like it, you can throw the whole OS off its game, and more problems occur.